1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition used as a material for a piezoelectric ceramic element such as a ceramic filter, a ceramic oscillator or the like.
2. Description of the Related Art
As a piezoelectric ceramic used for a ceramic filter or the like, a piezoelectric ceramic mainly composed of lead titanate zirconate (Pb (Ti.sub.X Zr.sub.1-X) O.sub.3) has been widely used, and a piezoelectric ceramic to which small amounts of various additives are added so as to improve the piezoelectric characteristics thereof has been used.
Particularly as a piezoelectric ceramic for a ceramic filter having flat group delay time (GDT) characteristics and causing low phase distortion, one having a small mechanical quality factor Qm has been required. As this piezoelectric ceramic having a small Qm value, a piezoelectric ceramic in which niobium oxide, antimony oxide, tantalum oxide and the like are added as additives to lead titanate zirconate (Pb (Ti.sub.X Zr.sub.1-X) O.sub.3) has been known.
However, many examples of the above described conventional piezoelectric ceramic composition having a small Qm value are those having a piezoelectric d constant increased for an actuator and those having an electromechanical coupling factor K increased for a wide band filter, so that they are insufficiently heat proof. Consequently, the conventional piezoelectric ceramic composition has the disadvantage in that where it is used as a surface mounted type ceramic filter element, if it is exposed to high temperatures in the reflow soldering process, the electromechanical coupling factor K thereof is significantly lowered, so that the filter characteristics thereof are largely degraded.
Furthermore, the above described piezoelectric ceramic composition having a small Qm value has the disadvantage in that it generally has low mechanical strength, so that it is liable to be cracked or chipped in the process of processing as a filter element.